Canada's Auditor General said that taxpayers are paying an extra half-billion dollars because of slow-decision-making in replacing the Champlain Bridge.

Michael Ferguson's report also indicates that the decision to have the bridge built via a public-private partnership could cost Canadians more than if the bridge had been built outright by the federal government.

The decision to replace the Champlain Bridge is examined in the fourth report issued Tuesday by the Auditor General.

In that report the AG found that engineers with the Crown corporation were concerned about the high maintenance costs for the bridge in the 1980s, just 25 years after the span was built.

In 1999 they noted that there was a good chance that part of the bridge would fail, and five years later engineers with the Jacques Cartier and Champlain Bridge Corporation determined the cost of maintaining the bridge would soon skyrocket.

However the knowledge that the Champlain was degrading quickly was not shared with Transport Canada, and so the federal government did not consider replacing the bridge until 2006-07, when the JCCBI said it would be cheaper to replace the bridge than to continue fixing it.

That prediction proved accurate.

While the annual cost of maintaining the bridge was about $10 million per year from 1999 until 2009, in subsequent years the cost jumped up to about $30 million per year, and the maintenance costs on the bridge reached $100 million in 2017.

The AG noted that it was only after five years of requests that the Harper government decided to build a new bridge. After two years of planning, the government decided to speed up the construction -- at an added cost -- because the old bridge was degrading so quickly.

Later that year, in 2013, an exterior girder on the old Champlain failed, requiring the installation of the "super-girder" that had been predicted.

The report notes that in addition to the extra construction costs, the traffic delays caused by frequent maintenance and the decision to divert heavy vehicles from the Champlain, were also imposing costs on citizens.

The report also says that the assumptions made to show a PPP would be cost-effective were overly optimistic about efficiency rates and that it severely underestimated how much it would cost to manage the project.

A union of public sector workers, the SCFP-Quebec, said it was not surprised by that finding.

The union's president, Denis Bolduc, said that he was skeptical when the estimated savings went from $227 million in one year to $1.75 billion the next.

The Auditor General said Infrastructure Canada should conduct regularly scheduled reviews of all facilities, take their life-cycle costs into effect, and plan to build replacement infrastructure before it is a dire necessity.

The new Champlain Bridge is scheduled to open on Dec. 21, 2018 at a budget of $4.2 billion.